Organic light emitting devices (OLED's), which make use of thin film materials that emit light when excited by electric current, are becoming an increasingly popular form of flat panel display technology for applications such as television sets, computer terminals, telecommunications equipment and a host of other applications. There are presently three predominant types of OLED construction: the "double heterostructure" (DH) OLED, the "single heterostructure" (SH) OLED, and the single layer polymer OLED. In the DH OLED, as shown in FIG. 1A, a substrate layer of glass 10 is coated by a thin layer of indium-tin-oxide (ITO) 11. Next, a thin (100-500 .ANG.) organic hole transporting layer (HTL) 12 is deposited on ITO layer 11. Deposited on the surface of HTL 12 is a thin (typically, 50 .ANG.-500 .ANG.) emission layer (EL) 13. The EL 13 provides the recombination site for electrons injected from a 100-500 .ANG. thick electron transporting layer 14 (ETL) with holes from the HTL 12. Examples of prior art ETL, EL and HTL materials are disclosed in U.S. Pat. No. 5,294,870, the disclosure of which is incorporated herein by reference.
Often, the EL 13 is doped with a highly fluorescent dye to tune color and increase the electroluminescent efficiency of the OLED. The device as shown in FIG. 1A is completed by depositing metal contacts 15, 16 and top electrode 17. Contacts 15 and 16 are typically fabricated from indium or Ti/Pt/Au. Electrode 17 is often a dual layer structure consisting of an alloy such as Mg/Ag 17' directly contacting the organic ETL 14, and a thick, high work function metal layer 17" such as gold (Au) or silver (Ag) on the Mg/Ag. The thick metal 17" is opaque. When proper bias voltage is applied between top electrode 17 and contacts 15 and 16, light emission occurs from emissive layer 13 through the glass substrate 10. An LED device of FIG. 1A typically has luminescent external quantum efficiencies of from 0.05% to 2% depending on the color of emission and the device structure.
The SH OLED, as shown in FIG. 1B, makes use of multifunctional layer 13 to serve as both EL and ETL. One limitation of the device of FIG. 1B is that the multifunctional layer 13 must have good electron transport capability. Otherwise, separate EL and ETL layers should be included as shown for the device of FIG. 1A.
A single layer polymer OLED is shown in FIG. 1C. As shown, this device includes a glass substrate 1 coated by a thin ITO layer 3. A thin organic layer 5 of spin-coated polymer, for example, is formed over ITO layer 3, and provides all of the functions of the HTL, ETL, and EL layers of the previously described devices. A metal electrode layer 6 is formed over organic layer 5. The metal is typically Mg or other conventionally used low work function metal.
OLED materials for the emission of red, green and blue light are known in the art. Conventional red OLEDS, however, are generally inferior to blue and green OLEDs in terms of brightness and efficiency. An example of a multicolor electroluminescent image display device employing organic compounds for light emitting pixels is disclosed in U.S. Pat. No. 5,294,870.